A clathrate is an inclusion compound in which the guest species are enclosed on all sides of the host species forming a framework lattice. Clathrate hydrates are a wide subgroup of clathrates, with the host framework made up of hydrogen bonded H2O molecules, and guest molecules trapped inside the polyhedral cages of the framework.
Dyadin et al first recognized classical hydrogen clathrate hydrate in 1999; see Dyadin, Y. A. et al., “Clathrate hydrates of hydrogen and neon”, Mendeleev Commun, 5, 209-210 (1999); and, Dyadin, Y. A. et al., “Clathrate formation in water-noble gas (hydrogen) systems at high pressures>>, J. Struct. Chem. 40, 790-795 (1999).
In 2002, Mao et al, found that hydrogen hydrate crystallizes in structure labeled as structure-II (sII) clathrate with a cubic symmetry and a lattice constant of a˜17.0 Å; see, Mao, W. L., et al., “Hydrogen Clusters in Clathrate Hydrate”, Science 297, 2247-2249 (2002). The crystal structure and precise composition of the hydrogen clathrate were determined by neutron diffraction as a function of pressure and temperature. The hydrogen occupancy in the clathrate (32+X)H2*136H2O, x=0-12 was found to change proportionally to changes in pressure and/or temperature, while leaving the host structure virtually intact. Hydrogen hydrates could form substituted phases with variable compositions.
Referring to FIG. 1, a H2—H2O phase diagram, synthesis of hydrogen clathrate hydrate has been performed using liquid water and hydrogen gas as the starting materials. U.S. Pat. No. 6,735,960, “Composition and Method for Hydrogen Storage”, issued May 18, 2004, by Mao et al., used diamond anvil cells (DAC) for preparation of clathrate. Mao et al. teaches the synthesis of hydrogen clathrate hydrates by cooling down water and hydrogen gas under a pressure of 1-6 kbar. The clathrate phase formed below 250 K during the cooling process. This process is indicated by horizontal arrow 10, traversing from Region I to Region III. Thus, Mao et al. teaches formation of clathrate directly from liquid water and hydrogen.
The present invention is a novel method where the formation of hydrogen clathrate hydrate occurs directly from ice and hydrogen, as indicated by vertical arrow 20, traversing from Region II to Region III.
Hydrogen clathrate hydrate presents a completely new technological means for hydrogen storage. Ice-like polyhedral cage frameworks of clathrate hydrate can hold substantial amounts of guest molecular hydrogen; up to 3.77% of total mass at atmospheric pressure and moderately low-temperatures (140 K˜200 K). Under higher pressures, the clathrates hydrate can hold greater amounts of guest molecular hydrogen. Note that the hydrogen storage capacity of clathrate hydrates is higher than the available hydrogen-storage metal hydrides, e.g. Mg2NiH4 (3.59%) and LaNi5H6 (1.37%). A significant benefit of hydrogen clathrate hydrate is that by controlling pressure and temperature, it is possible to reversibly trap and/or release substantial amounts of molecular hydrogen.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.